The invention relates to bar code scanning devices and particularly to those scanners utilizing a rotating mirror. The invention adapts a DC brushless motor for use in rotating a mirror of a scanner. with a number of attendant advantages.
In the prior art, laser scanners, particularly POS (point of sale) retail scanners, have used AC synchronous shaded pole type motors for driving a rotating element such as a multifacated rotating polygon mirror. Typically these 60-cycle synchronous motors were inefficient and generated a considerable amount of heat. This was often deleterious to the rotor bearings and the motor spindle, which were sometimes the first components to wear out in such prior scanners.
A safety requirement for laser scanning devices is that the laser beam must not be operative when the scanner motor is stopped, or rotating below a certain speed which will assure that retina exposure will be of sufficiently short duration to be safe for persons positioned nearby. Thus, if a scanner motor failed or was below speed, the safety requirement has been that the laser beam must be immediately shut off.
In the prior art, this requirement typically was met through the use of a windswitch, i.e. a mechanical switch with a wind-operated flap which would keep the windswitch in the "on" position only so long as wind or air movement generated by the laser scanner motor (with wind-producing blades) was at or above a minimum speed. In addition to the difficulty in precisely defining such a minimum speed with these relatively crude, spring-loaded wind switches, they were also relatively costly and took up space in the laser scanner.
Laser scanner motors and associated circuitry of the prior art have not had the advantages of efficiency, low temperature rise, low noise, reliability and simplicity of components, and reduced space requirements as in the present invention described below.